As has been known, developing processes in electrophotography use one-component developers containing a toner as the only main component or use two-component developers containing a carrier and a toner in a mixed state. The developing processes using the two-component developers are advantageous over those using the one-component developers in that a high-quality image can be consistently formed for a long period of time, since the two-component developers contain the powdery carrier providing a large area for frictionally charging the toner and provide excellent charge rising property and charge stability. This is because the powdery carrier has a specific surface area remarkably larger than the surface area of a charging sleeve commonly used in the developing processes using the one-component developers, increasing the chance of the contact between the carrier and the toner. For the above reasons, developing processes using two-component developers are employed in digital electrophotographic systems where a latent electrostatic image is formed on a photoconductor using, for example, a laser beam and then the formed latent electrostatic image is visualized.
In an attempt to increase resolution and highlight reproducibility and to respond to formation of color images, the recent interest has focused on formation of a high-density latent image having a minimum unit (1 dot) which is as small as possible. In view of this, keen demand has arisen for development systems where such a latent image (dot) can be developed with fidelity. Under such circumstances, various attempts have been made to find out optimum process conditions and to desirably modify developers; e.g., toners and carriers. Regarding process conditions, it is advantageous that, for example, the developing gap is made to be small, photoconductors are made to be thin and a writing beam diameter is made to be small. However, these measures pose serious problems such as cost elevation and degradation of reliability.
Also, use of toners having a small particle diameter remarkably improves dot reproducibility, but developers containing such toners problematically cause, for example, background smear, insufficient image density and toner spent on carriers. As compared with black toners, full-color toners, which is used in combination with a resin having a low softening point for attaining sufficient color tone, cause considerable toner spent on the carriers to degrade the developers, resulting in easily causing toner scattering and background smear.
Various Patent Literatures disclose use of a carrier having a small particle diameter. For example, Patent Literature 1 discloses a developing method including reversely developing, in an applied bias electric field formed of AC and DC components at a developing section, a latent electrostatic image formed on a latent image bearing member containing an organic photoconductive layer, using a magnetic brush of a two-component developer containing a carrier and a toner borne on a developer bearing member. In this method, the toner has the same charge polarity as the latent electrostatic image; and the carrier contains a carrier core having ferrite particles and an electrical insulating resin applied thereon in an amount of 0.1% by mass to 5.0% by mass with respect to the carrier core, and has a weight average particle diameter of 30 μm to 65 μm and an average pore size thereon of 1,500 angstrom to 30,000 angstrom.
Patent Literature 2 discloses an electrophotographic carrier having a 50% average particle diameter (D50) of 15 μm to 45 μm, the electrophotographic carrier containing carrier particles having a particle diameter of 22 μm or smaller in a ratio of 1% to 20%, carrier particles having a particle diameter of 16 μm or smaller in a ratio of 3% or less, carrier particles having a particle diameter of 62 μm or larger in a ratio of 2% to 15% and carrier particles having a particle diameter of 88 μm or larger in a ratio of 2% or less, wherein a specific surface area S1 as measured by an air permeation method and a specific surface area S2 calculated by the equation S2=(6/ρ·D50)×104 (wherein ρ denotes a specific gravity of the carrier) satisfy the relation 1.2≦S1/S2≦2.0.
Patent Literature 3 discloses a carrier used in a developer for developing a latent electrostatic image, the carrier having a 50% volume average particle diameter (D50) of 30 μm to 80 μm, having a ratio of the 50% volume average particle diameter to a 10% volume average particle diameter (D50/D10) of 1.8 or lower, having a ratio of a 90% volume average particle diameter to the 50% volume average particle diameter (D90/D50) of 1.8 or lower, containing carrier particles having a volume particle diameter of 20 μm or less in a ratio less than 3% and having a magnetization of 52 emu/g to 65 emu/g at 1 kOe.
Use of such small carrier having a large surface area exhibits advantageous effects as described below:
(1) each toner particles can be sufficiently frictionally-charged to reduce toner particles having a low charging amount and reversely charged toner particles, resulting in that background smear is less likely to occur and excellent dot reproducibility can be attained (i.e., less toner scattering and bleeding);(2) the average charging amount of toner particles can be decreased to form an image having a sufficient image density:(3) when it is used in combination with toner particles having a small particle diameter, the coverage of the carrier with the toner particles is not high, resulting in avoiding failures caused by using such toner particles and making them to exhibit their advantageous effects; and(4) a dense magnetic brush whose toner particle chains have excellent flowability is formed to reduce, on the formed image, trails of the chains.
However, when such carrier having a small particle diameter is produced with the conventionally known production method employing, as a liquid droplet forming section, a rotating disc or a two-fluid nozzle, the particle size distribution of the formed liquid droplets is problematically very broader than that of the carrier of interest. Thus, classification must be repeatedly performed for producing the target small carrier, and in general, the production yield is decreased to as low as several tens percent.
In an attempt to overcome the aforementioned problems occurring during production of such small carrier, Patent Literatures 4 and 5 disclose a vibrating-orifice granulator and an ink-jet granulator, respectively. In these granulators, a carrier composition liquid is discharged from nozzles having a pore size smaller than the size of the formed liquid droplets. Thus, nozzle clogging often occurs which is caused by foreign matter (e.g., dust) and/or aggregates of magnetic powder contained in the carrier composition liquid. In order to avoid this problem, there is provided an additional step for increasing dispersibility of a slurry containing magnetic powder. In addition, filtration is repeatedly performed and/or a cleaning mechanism for nozzles is provided. None of these measures have attained satisfactorily reliable carrier-production.
With reference to FIG. 1, next will be briefly described a vibrating-orifice granulator based on the principle of liquid droplet formation described in Patent Literature 4.
This apparatus includes a housing 501, an opening 502 formed in the housing 501, a nozzle plate 503 having nozzles (openings) (serving as a discharge member), a flow passage member 504 screwed on the housing 501, an O-ring 505, a flow passage 506 provided in the flow passage member 504, an insulating support 507, a hollow counter electrode 508 and a DC power source 509, the nozzle plate 503 facing the opening 502 and being secured via the O-ring 505 by the end surface of the flow passage member 504. With this configuration, when the nozzle plate 503 is vibrated by an unillustrated vibration generating unit, a slurry fed through the flow passage 506 is discharged downwardly in a form of liquid droplet from the nozzles of the nozzle plate 503. Notably, the granulator disclosed in Patent Literature 5 is called a continuous ink-jet granulator, which is the same in principle as the vibrating-orifice granulator disclosed in Patent Literature 4.
Also, below the nozzle plate 503 is provided the hollow counter electrode 508 secured by the insulating support 507. A DC high voltage is applied to the hollow counter electrode 508 from the DC power source 509. Further, dispersing gas 511 is fed through the gap between the support 507 and the housing 501 toward an underside surface of the nozzle plate 503, and the slurry is discharged downstream from the nozzle plate 503 as liquid droplets 510 through the counter electrode 508.
A carrier production method using the above-described vibrating-orifice (continuous ink-jet) granulator disclosed in Patent Literatures 4 and 5 can produce carrier having a sharp particle size distribution, which carrier has been demanded for avoiding carrier adhesion.
However, a carrier composition liquid containing aggregated particles easily causes nozzle clogging, making it difficult to continue particle formation for a long period of time. In other words, when a slurry containing magnetic powder in a dispersed state is discharged from nozzles having a small pore size using the conventional apparatuses based on vibrating-orifice (continuous ink-jet) granulation, nozzle clogging often occurs and it is difficult to continuously produce particles over a long period of time.    Patent Literature 1: Japanese Patent (JP-B) No. 2832013    Patent Literature 2: JP-B No. 3029180    Patent Literature 3: Japanese Patent Application Laid-Open (JP-A) No. 10-198077    Patent Literature 4: JP-A No. 2007-171499    Patent Literature 5: JP-A No. 2007-216213